A teachable moment (TM) refers to life events or transitions, such as physician visits, early detection screening feedback, pregnancy, and hospitalization for, and diagnosis of, disease, that inspire health behavior change. TMs have guided the implementation of nicotine addiction treatments at a time-point or with a population to enhance intervention acceptance and efficacy. Despite strong inuitive appeal of the TM concept and over 150 studies of interventions implemented in a TM, there has been little investigation of the influence of TMs for smoking, particularly for promoting smoking cessation among those indirectly affected by a TM (e.g., relatives of cancer patients). Determining the influence of TMs for smoking cessation has been difficult since past studies have not: 1) compared TM quit rates to matched controls not experiencing a TM;2) examined longitudinal changes in smoking after a TM;and 3) assessed moderators and mediators of TMs. To enhance understanding of TMs for smoking cessation, we will conduct a prospective observational study using a matched-group design to assess whether a lung cancer diagnosis increases enrollment in a smoking cessation program and cessation, and reduces smoking, among the patient's relatives/spouse who smoke. We will compare enrollment in a cessation program and cessation and smoking rates in 2 groups of smokers: 1) relatives/spouses of newly-diagnosed lung cancer patients, and 2) relatives/spouses of patients with an orthopedic condition. We will also assess moderators and mediators of the link between a lung cancer diagnosis and enrollment, cessation, and smoking. The primary aim is to compare relatives/spouses of lung cancer patients to relatives/spouses of orthopedic patients in rates of enrollment in a smoking cessation program, cessation, and smoking. We will also explore: 1) a heuristic that TMs influence smoking by increasing perceptions of personal risk and positive outcome expectancies, prompting strong affective responses, and by redefining self-concept and social role;and 2) characteristics about a TM and the person exposed to the TM that may influence the impact of the TM on smoking, including: perceived seriousness of the illness, closeness between the relative and patient, and the relative/spouse's coping and attribution style. This study could clarify the TM concept for smoking cessation, shed light on the underlying mechanisms of TMs, and guide future development and implementation of TM-based smoking cessation interventions.